1. Field of the Invention
The invention relates to a combined cycle power plant with a gas turbine circuit and a steam turbine circuit, and more particularly, to a combined cycle power plant wherein the exhaust gases from a gas turbine transfer residual heat to a steam turbine via the working medium flowing in a once through steam generator.
2. Background of the Invention
The water/steam circuit of current combined power plants is operated, without exception, on the basis of subcritical parameters. As a rule, the heat recovery steam generator for utilizing the waste heat from the gas turbines is designed with a drum boiler, with a once through boiler or with a combination thereof. In the case of large highly efficient plants, multiple pressure plants with reheating are sometimes used. However, in comparison with conventionally fired boilers, the flue gas temperature in the case of heat recovery steam generators is limited. In particular, the evaporation which occurs at a constant temperature leads to thermodynamic and technical design constraints.
At the present time, heat recovery steam generators for utilizing the waste heat from gas turbine plants are designed, as a rule, with drum boilers. The steam circuit is cleaned by upgrading in the respective drum and by continuous or discontinuous blowdown of the drum. Upgrading occurs because the water evaporates in the boiler drum. Nonvolatile substances therefore remain in the boiler water and are increasingly upgraded. With the blowdown of the boiler water, the substances are consequently efficiently removed in concentrated form from the circuit. Moreover, in many instances, a solid alkalizing agent, such as trisodiumphosphate or sodium hydroxide, is added to the drum water in order to adjust the pH value in the boiler.
However, if the heat recovery steam generators are provided with a simple once through boiler, this cleaning mechanism is not used, since such a boiler cannot be blown down. The cleaning of the water/steam circuit is carried out, in this case, in a condensate polishing plant, in which the condensate is filtered, before being introduced into the steam generator, and, if appropriate, is additionally desalinated by means of ion exchangers.
However, in both types of boilers, one problem is that many undesirable substances, such as, chlorides and sulfates, are present in a volatile form, for example, HCl or H.sub.2 SO.sub.4. This also applies to conventional circuit conditioning with ammonia in the form of volatile ammonium chlorides or of ammonium sulfates. Such materials may lead to corrosion and consequently to operating faults and damage.
A heat recovery steam generator working with a drum boiler in a low-pressure system and with a once through boiler in the high-pressure system is known from EP-A1-0,359,735. In order to make the plant simpler and more efficient, however, in the disclosed plant the drum also performs the function of the feedwater tank/deaerator, such that the steam drum is provided with integrated deaeration.